1. Technical Field to which the Invention Belongs
The present invention relates to a reduction casting method in which casting is performed while an oxide film formed on a surface of molten metal at the time of casting is reduced.
2. Prior Art
There are various types of casting methods such as a gravity casting method (GDC), a low pressure die casting method (LPDC), a die casting method (DC), a squeeze casting method (SC), a thixomolding method and the like. All of these methods perform casting by pouring molten metal into a cavity of a molding die thereby molding it into a predetermined shape. In these casting methods, it has been a problem that, in a method among these casting methods in which an oxide film is likely to be formed on a surface of the molten metal, for example, at aluminum casting or the like, a surface tension of the molten metal is increased by the oxide film formed on the surface of the molten metal to deteriorate a flowing property, a running property and an adhesive property of the molten metal thereby causing casting imperfections such as insufficient filling, a surface fold and the like.
The present invention is attained in order to solve these problems and has an object to provide a reduction casting method which is capable of performing favorable casting by reducing an oxide film formed on a surface of the molten metal.
Further, it is an object of the present invention to provide a reduction casing method by which a cast product having an excellent appearance can be produced in an easy manner and also constitution of a casting apparatus can be simplified.
As a method to solve these problems, the present applicant has developed a method of performing casting by a reduction casting method while an oxide film formed on a surface of molten metal of aluminum is reduced. In this reduction casting method, a magnesium-nitrogen compound (Mg3N2) having a strong reducing property is prepared by using a nitrogen gas and a magnesium gas and, then, casting is performed while the thus-prepared magnesium-nitrogen compound is allowed to act on the molten metal of aluminum to reduce the oxide film formed on the surface of the molten metal. By pouring the molten metal into a cavity of a molding die in a state in which the magnesium-nitrogen compound is deposited on a surface of the cavity of the molding die, when the molten metal comes into contact with the surface of the cavity, the oxide film formed on the surface of the molten metal is reduced to decrease a surface tension of the molten metal thereby enhancing a flowing property and a wetting property of the molten metal whereupon a cast product which does not have a cast imperfection but has an excellent appearance deprived of a surface fold or the like can easily be produced.
The reduction casting method is characterized in that casting is performed by allowing a reducing compound such as a magnesium-nitrogen compound to act on molten metal to reduce an oxide film formed on a surface of the molten metal. To this end, at the time of performing casting, a magnesium metal and a nitrogen gas are reacted with each other to prepare a magnesium-nitrogen compound and, then, the thus-prepared magnesium-nitrogen compound is allowed to act on the molten metal. As a method of preparing the magnesium-nitrogen compound, there are one method in which the magnesium-nitrogen compound is prepared in advance in a furnace or the like arranged separately from a molding die and the other method in which the nitrogen gas and a magnesium gas are each individually introduced inside the cavity and, then, the magnesium-nitrogen compound is prepared in the cavity.
In either method, the magnesium metal is heated to allow it to be a magnesium gas and, then, the thus-prepared magnesium gas is allowed to react with the nitrogen gas to prepare the magnesium-nitrogen compound. Because of an extremely strong reducing property of the magnesium-nitrogen compound, it is necessary that the magnesium-nitrogen compound is treated under a non-oxidizing atmosphere at both stages of preparing it and of allowing it o act on the molten metal. While, in a conventional reduction casting method, a metallic gas and the nitrogen gas are used as in a case in which the magnesium gas and the nitrogen gas are r acted with each other to prepare the magnesium-nitrogen compound. As described above, it is necessary in the reduction casting method that the reducing property of the reducing compound i not impaired whereupon much attention must be paid in casting operation compared with an ordinary casting apparatus. Therefore, it is desirous that, in a case in which a constitution or the like of the apparatus can be as simple as possible, not only the constitution of the apparatus can be simplified, but also the casting operation can be conducted in a convenient manner.
The present invention is made to achieve the above-mentioned desires thus found by the inventor.
That is, the afore-mentioned desires can be achieved by a reduction casting method for performing casting while an oxide film formed on a surface of molten metal is reduced, according to the present invention, comprising the steps of:
allowing an inside of a cavity of a molding die to be in a non-oxidizing atmosphere;
allowing a reducing substance having a stronger reducing property than a metal of the molten metal has to act on the molten metal; and
performing casting while the oxide film formed on the surface of the molten metal is reduced.
Further, according to present invention, it is preferable that the reducing substance is transferred by a carrier gas that does not react with the reducing substance to allow the reducing substance to act on the molten metal.
Further, according to the present invention, as a method of allowing the inside of the cavity of the molding die to be in the non-oxidizing atmosphere, it is preferable that the carrier gas that does not react with the reducing substance is introduced into the inside of the cavity to replace an acidic atmosphere inside the cavity therewith.
Further, according to the present invention, as a method of allowing the inside of the cavity of the molding die to be in the non-oxidizing atmosphere, it is preferable that the inside of the cavity is subjected to vacuum suction.
Further, according to the present invention, it is preferable that a metallic gas is used as the reducing substance.
Further, according to the invention, there is provided the reduction casting method, in which favorable aluminum casting can be performed by using the molten metal of aluminum or an alloy thereof as the molten metal and using a magnesium gas as the reducing substance.
Further, according to the invention, an argon gas is favorably used as a carrier gas of the magnesium gas.